The invention relates to the field of quantum computing, and particularly to superconducting quantum computing.
Mesoscopic superconducting systems have received attention as systems exhibiting complex physical phenomena for many years. Recently, these phenomena have been understood to have practical application in the field of quantum computing. See, e.g., A. Assime, G. Johansson, G. Wendin, R. Schoelkopf, and P. Delsing, xe2x80x9cRadio-Frequency Single-Electron Transistor as Readout Device for Qubits: Charge Sensitivity and Backactionxe2x80x9d, Phys. Rev. Lett. 86, p. 3376 (Apr. 2001), and the references cited therein, and Alexandre Zagoskin, U.S. patent application Ser. No. 09/452749, entitled xe2x80x9cPermanent Readout Superconducting Qubitxe2x80x9d, filed Dec. 1, 1999, which are herein incorporated by reference in their entirety.
Quantum computing generally involves initializing the states of N qubits (quantum bits), creating controlled entanglements among them, allowing these states to evolve, and reading out the states of the qubits after the evolution. A qubit is a quantum bit, the counterpart in quantum computing to the binary digit or bit of classical computing. Just as a bit is the basic unit of information in a classical computer, a qubit is the basic unit of information in a quantum computer. A qubit is conventionally a system having two degenerate (e.g., of equal energy) quantum states, wherein the quantum state of the qubit can be in a superposition of the two degenerate states. The two degenerate states are also referred to as basis states. Further, the two degenerate or basis states are denoted |0 greater than  and |1 greater than . The qubit can be in any superposition of these two degenerate states, making it fundamentally different from a bit in an ordinary digital computer. If certain conditions are satisfied, N qubits can define an initial state that is a combination of 2N classical states. This initial state undergoes an evolution, governed by the interactions that the qubits have among themselves and with external influences, providing quantum mechanical operations that have no analogy with classical computing. The evolution of the states of N qubits defines a calculation or, in effect, 2N simultaneous classical calculations (e.g. conventional calculations as in those performed using a conventional computer). Reading out the states of the qubits after evolution completely determines the results of the calculations.
To appreciate the conditions necessary for N qubits to represent a combination of 2N classical states, the principles of superposition and entanglement must be introduced. Superposition may be described by considering a qubit as a particle in a magnetic field. The particle""s spin may be either in alignment with the field, which is known as a spin-up state, or opposite to the field, which is known as a spin-down state. Changing the particle""s spin from one state to another is achieved by using a pulse of energy, such as from a laser. If it takes one arbitrary unit of laser energy to change the particle""s spin from one state to another, the question arises as to what happens if only a half a unit of laser energy is used and the particle is completely isolated from all external influences. According to quantum mechanical principles, the particle then enters a superposition of states, in which it behaves as if it were in both states simultaneously. Each qubit so utilized could take a superposition of both 0 and 1. Because of this property, the number of states that a quantum computer could undertake is 2n, where n is the number of qubits used to perform the computation. A quantum computer comprising 500 qubits would have a potential to do 2500 calculations in a single step. Conventional digital computers cannot perform calculations on a scale that even approaches 2500 calculations in any reasonable period of time. In order to achieve the enormous processing power exhibited by quantum computers, the qubits must interact each with each other in a manner that is known as quantum entanglement (entanglement).
Qubits that have interacted with each other at some point retain a type of connection and can be entangled with each other in pairs, in a process known as correlation. When a first qubit is entangled with a second qubit, the quantum states of the first and second qubits become correlated quantum mechanically. Entanglement is a quantum computing operation that has no analogue in classical computing. Once a pair of qubits has-been entangled, information from only one of the qubits necessarily effects the state of the other qubit and vice versa. For example, once a pair of qubits are entangled, operations performed on one of the pair will simultaneously effect both qubits in the pair. Quantum entanglement allows qubits that are separated by larger distances to interact with each other instantaneously (not limited to the speed of light). No matter how great the distance between the correlated particles, they will remain entangled as long as they are isolated.
Taken together, quantum superposition and entanglement create an enormously enhanced computing power. Where a 2-bit register in an ordinary computer can store only one of four binary configurations (00, 01, 10, or 11) at any given time, a 2-qubit register in a quantum computer can store all four numbers simultaneously, because each qubit represents two values. If more qubits are entangled, the increased capacity is expanded exponentially.
Several physical systems have been proposed for the qubits in a quantum computer. One system uses molecules having degenerate nuclear-spin states. See N. Gershenfeld and I. Chuang, xe2x80x9cMethod and Apparatus for Quantum Information Processing,xe2x80x9d U.S. Pat. No. 5,917,322, which is herein incorporated by reference in its entirety. Nuclear magnetic resonance (NMR) techniques can read the spin states. These systems have successfully implemented a search algorithm, see, e.g., M. Mosca, R. H. Hansen, and J. A. Jones, xe2x80x9cImplementation of a quantum search algorithm on a quantum computer,xe2x80x9d Nature 393, 344 (1998) and references therein, and a number-ordering algorithm, see, e.g., L. M. K. Vandersypen, M. Steffen, G. Breyta, C. S. Yannoni, R. Cleve, and I. L. Chuang, xe2x80x9cExperimental realization of order-finding with a quantum computer,xe2x80x9d preprint quant-ph/0007017, which is herein incorporated by reference in its entirety, and references therein. The number-ordering algorithm is related to the quantum Fourier transform, an essential element of both Shor""s factoring algorithm (P. Shor, 1994, Proc. 35th Ann. Symp. On Found. Of Comp. Sci., pp. 124-134, IEEE Comp. Soc. Press, Los Alamitos, Calif.) and Grover""s algorithm for searching unsorted databases (Grover, 1997, Phys. Rev. Lett. 78, p. 325), which is herein incorporated by reference in its entirety. However, expanding such systems to a commercially useful number of qubits is difficult. More generally, many of the current proposals will not scale up from a few qubits to the 102xcx9c103 qubits needed for most practical calculations.
Unfortunately, current methods for entangling qubits are susceptible to loss of coherence. Loss of coherence is the loss of the phases of quantum superpositions in a qubit as a result of interactions with the environment. Thus, loss of coherence results in the loss of the superposition of states in a qubit. See, for example, Zurek, 1991, Phys. Today 44, p. 36; Leggett et al., 1987, Rev. Mod. Phys. 59, p. 1; Weiss, 1999, Quantitative Dissipative Systems, 2nd ed., World Scientific, Singapore; Hu et al; arXiv:cond-mat/0108339, which are herein incorporated by reference in their entirety. Entanglement of quantum states of qubits can be an important step in the application of quantum algorithms. See, for example, P. Shor, SLAM J. of Comput., 26:5, 1484-1509 (1997), which is herein incorporated by reference in its entirety. Current methods for entangling phase qubits require interaction of the flux in each of the qubits, see Yuriy Makhlin, Gerd Schon, Alexandre Shnirman, xe2x80x9cQuantum state engineering with Josephson-junction devices,xe2x80x9d LANL preprint, cond-mat/0011269 (November 2000), which is herein incorporated by reference in its entirety. This form of entanglement is sensitive to the qubit coupling with surrounding fields, which cause decoherence and loss of information.
As discussed above, currently proposed methods for read out, initialization, and entanglement of a qubit involve detection or manipulation of magnetic fields at the location of the qubit, which make these methods susceptible to decoherence and limits the overall scalability of the resulting quantum computing device. Thus, there is a need in the art for methods of entangling and otherwise controlling qubits. Such methods can be used to build efficient quantum registers where decoherence and other sources of noise are minimized but where scalability is improved.
The present invention addresses the need in the art for entangling and controlling qubits. The methods and apparatus of the present invention are based on the unexpected discovery of a way to entangle a charge device, such as a charge qubit, with a phase device, such as phase qubit, in such a manner that the charge device can be used to read out the quantum state of the phase device. In the present invention, the phase device and the charge device are connected by a xcfx80/2 phase shift device. Without intending to be limited to any particular theory, it is believed that the xcfx80/2 phase shift device pushes the circuit, which includes the phase device and the charge device, into a point of operation where current in the charge device is maximally sensitive to changes in the flux in the phase device. Because of this phenomenon, the charge device can read out the quantum state of the phase device.
One embodiment of the present invention provides a superconducting structure comprising a mesoscopic phase device, a mesoscopic charge device, and a mechanism for coupling the mesoscopic phase device and the mesoscopic charge device so that the quantum mechanical state of the mesoscopic phase device and the quantum mechanical state of the mesoscopic charge device interact. In some embodiments, the mesoscopic phase device includes a superconducting mesoscopic island that is characterized by a charging energy EC and a Josephson coupling energy EJ. Further, the charging energy EC of the mesoscopic island is large compared with the Josephson coupling energy EJ. That is, EC is at least 10 times greater to about 100 times greater than EJ.
In some embodiments of the present invention, the mesoscopic phase device includes a superconducting mesoscopic island and the charging energy EC of the mesoscopic island is on the same order as the Josephson coupling energy EJ of the island. In some embodiments of the present invention, the mesoscopic phase device is composed of a superconducting material that violates time reversal symmetry. Materials that violate time reversal symmetry include d-wave superconductors, such as YBa2Cu3O7xe2x88x92x, Bi2Sr2Canxe2x88x921CunO2n+4, Tl2Ba2CuO6+x, and HgBa2CuO4, and p-wave superconductors, such as Sr2RuO4 or CeIrIn5.
In still other embodiments of the present invention, the mesoscopic phase device includes a mesoscopic island, a bulk region; and a clean Josephson junction, separating said mesoscopic island from the bulk region. A clean Josephson junction is a Josephson junction that is free of scattering sites. In yet other embodiments of the invention, the mesoscopic phase device includes an inner superconducting loop, which includes one or more Josephson junctions, and an outer superconducting loop, which includes two or more Josephson junctions. Further, the inner superconducting loop is inductively coupled to the outer superconducting loop. In some embodiments, the inner superconducting loop of the mesoscopic phase device is made of a conventional superconducting material such as aluminum (Al), niobium (Nb), and lead (Pb). In other embodiments, the inner superconducting loop is made of a superconducting material that violates time reversal symmetry. In some embodiments of the invention, the outer superconducting loop is made of a conventional superconducting material such as aluminum (Al), niobium (Nb), and lead (Pb). In some embodiments of the present invention, the outer superconducting loop includes a phase shift device, such as a xcfx80/2 phase shift device.
In still other embodiments of the present invention, the mesoscopic charge device includes a mesoscopic island that is characterized by a charging energy EC and a Josephson coupling energy EJ. The charging energy EC of the mesoscopic superconducting region is small compared with the Josephson coupling energy EJ.
In some embodiments of the present invention, the mesoscopic charge device includes device leads, a mesoscopic island, and two Josephson junctions connected to the device leads. The two Josephson junctions are coupled to the mesoscopic island thereby isolating the mesoscopic island from the device leads. The mesoscopic charge device further includes an electrode capacitively coupled to the mesoscopic island and a mechanism that controls the charge on the capacitively coupled electrode.
In some embodiments of the present invention, the mechanism for coupling includes a mechanism for introducing a phase shift. In some embodiments the mechanism for introducing a phase shift is a xcfx80/2 phase shift device such as a phase shift Josephson junction. In some embodiments of the present invention, the phase shift Josephson junction includes a ferromagnetic material that is placed between the leads of the phase shift Josephson junction. In some embodiments, the phase shift Josephson junction includes an unconventional superconducting material (e.g., wave or p-wave) and the leads of the phase shift Josephson junction are connected across a grain boundary Josephson junction so that a phase shift is accumulated in transition across said grain boundary. In some embodiments of the present invention, the angle of crystal misorientation across said grain boundary is 0xc2x0 to 45xc2x0 with respect to the angle of the grain boundary. In some embodiments, the mechanism for coupling the mesoscopic phase device and the mesoscopic charge device coherently couples the mesoscopic phase device and the mesoscopic charge device. In some embodiments of the present invention, the mechanism for coupling the mesoscopic phase device and the mesoscopic charge device is capable of entangling the mesoscopic phase device and the mesoscopic charge device.
Another aspect of the invention provides a superconducting structure comprising a mesoscopic phase device, a mesoscopic charge device, and a mechanism for coupling the mesoscopic phase device to the mesoscopic charge device so that the state of the mesoscopic phase device and the state of the mesoscopic charge device interact. Further, the invention provides a mechanism for reading out the state of the mesoscopic charge device. In some embodiments, the mesoscopic charge device includes device leads, and the mechanism for reading out the state of the mesoscopic charge device includes a shunt resistor, a current source, a mechanism for measuring the voltage drop across the leads of the mesoscopic charge device, and a mechanism for grounding the leads of the mesoscopic charge device. The shunt resistor, current source, mechanism for measuring the voltage drop, and the mechanism for grounding are in parallel with the device leads of the mesoscopic charge device.
Yet another aspect of the invention provides a superconducting structure that includes a mesoscopic phase device, a mesoscopic charge device, a mechanism for coupling the mesoscopic phase device to the mesoscopic charge device such that the state of the mesoscopic phase device and the state of the mesoscopic charge device interact, and a mechanism for reading out the state of the mesoscopic phase device. In some embodiments, the mesoscopic charge device include leads and the mechanism for reading out the state of the mesoscopic charge device includes a shunt resistor, a current source, a mechanism for measuring the voltage drop across the leads of the mesoscopic charge device, and a mechanism for grounding the leads of the mesoscopic charge device. In such embodiments, the shunt resistor, current source, mechanism for measuring the voltage drop, and the mechanism for grounding are placed in parallel with the leads of the mesoscopic charge device.
Still another aspect of the invention provides a superconducting structure that includes a plurality of mesoscopic phase devices. Each mesoscopic phase device in the plurality of mesoscopic phase devices is placed in parallel. The superconducting structure further includes a mesoscopic charge device, a mechanism for coupling a first mesoscopic phase device in the plurality of mesoscopic phase devices and the mesoscopic charge device so that the state of the first mesoscopic phase device and the state of the mesoscopic charge device interact, and a mechanism for reading out the state of said mesoscopic charge device. In some embodiments, each mesoscopic phase device in the plurality of mesoscopic phase devices is a phase qubit and the superconducting structure forms a quantum register.
Some embodiments of the present invention provide a superconducting structure. The structure includes a plurality of mesoscopic phase devices and a plurality of mesoscopic charge devices. Each mesoscopic phase device in the plurality of mesoscopic phase devices is placed in parallel. The structure further provides a mechanism for coupling a first mesoscopic phase device in the plurality of mesoscopic phase devices and a first mesoscopic charge device in the plurality of mesoscopic charge devices in such a manner that the state of the specific first phase device and the state of the first mesoseopic charge device interact. The structure further provides a mechanism for reading out the state of the first mesoscopic charge device or said mesoscopic phase device. In some embodiments, each mesoscopic phase device in the plurality of mesoscopic phase devices and each mesoscopic charge device in the plurality of mesoscopic charge devices is a qubit. Further, the superconducting structure forms a heterogeneous quantum register.